1. Field of the Invention
The present invention relates to the field of combustion equipment, and more particularly but not by way of limitation, to a burner assembly which substantially reduces the nitrogen oxide content of a flue gas effluent from a furnace and the like.
2. Discussion
Oxides of nitrogen are contaminants emitted during the combustion of industrial fuels. In every combustion process, where nitrogen is present, the high temperatures result in the fixation of some oxides of nitrogen. These compounds are found in flue gases mainly as nitric oxide (NO), with lesser amounts of nitrogen dioxide (NO.sub.2) and other oxides. Since nitric oxide continues to oxidize to nitrogen dioxide in air at ordinary temperatures, the total amount of nitric oxide plus nitrogen dioxide in a flue gas effluent is referred to simply as nitrogen oxides, or NO.sub.x, and expressed as NO.sub.2.
Emissions of nitrogen oxides from stack gases, through atmospheric reactions, produce "smog". The amount of NO.sub.x in vented gases is regulated by various state and federal agencies, especially in such congested areas as that of the Los Angeles Basin in the State of California. Recent rules of the South Coast Air Quality Management District of that state decree that NO.sub.x emissions cannot exceed 0.03 lbs/MM BTUs, roughly 25 ppm, (parts per million by volume dry), a NO.sub.x level which is below that permitted previously.
Tightening state and federal emission requirements have lead to considerable effort to find ways to remove or prevent the formation of nitrogen oxides in combustion processes so that such gases may be discharged to the atmosphere without further deleterious effect on the environment. Generally, prior art treatment NO.sub.x control has involved two methods. The first is that of the treatment of combustion products, sometimes referred to as post combustion treatment.
One such post combustion treatment for removing nitrogen oxides utilizes an absorption medium to absorb the oxides of nitrogen. However, this method results in the formation of either an acidic liquid or other nitrogen containing noxious liquid streams which must be treated further before safe discharge to the environment.
Other post combustion treatments for removing NO.sub.x have employed catalysts in combination with ammonia injection for selective catalytic reduction (SCR) of NO.sub.x from gaseous effluents. Still other non-catalytic processes have employed ammonia, ammonium formate, ammonium oxalate, ammonium carbonate and the like for selectively reducing NO.sub.x content of gaseous effluents. These injection technologies are limited by the reaction kinetics of the injected chemicals; furthermore, such treatments result in undesirable emissions not created by the combustion process, such as ammonia break through and the like.
Another prior art process for reducing NO.sub.x employs the concept of reducing NO.sub.x in the presence of an excess of a hydrocarbon at elevated temperatures. This process reduces the amount of NO.sub.x in the combustion gases, but products such as carbon monoxide, hydrogen, hydrocarbons and particulate carbon, are produced in such quantities that the release of the gases containing these products is prohibitive until additional steps are taken to further treat the gases. U.S. Pat. No. 3,873,671 issued to Reed et al.. provides for the burning of a hydrocarbon fuel with less than the stoichiometric amount of oxygen. Combustion products are provided an excess of oxidizable material under conditions that reduce the NO.sub.x content, and are then cooled to between about 1200.degree. F. to 2000.degree. F. with a fluid which is substantially free of oxygen. To prevent venting excess combustibles into the atmosphere, the cooled mixture of nitrogen, combustion products and other oxidizable materials is thereafter combusted in a second zone with sufficient oxygen to oxidize substantially all of the oxidizable combustion products while minimizing the oxides of nitrogen. This process achieves NO.sub. x emission reduction to about 50 to 100 ppm.
The second method of dealing with NO.sub.x control is that of the prevention of NO.sub.x formation in a combustion process. One such method is external flue gas recirculation in which a portion of the flue gas created by a combustion process is mixed with the inlet air fed to the burner. An example is found in U.S. Pat. No. 4,445,843 issued to Nutcher which taught a low NO.sub.x burner in which flue gas effluent is recirculated to be mixed with combustion air fed to the burner of a furnace. This system, while working in the prevention of NO.sub.x formation, requires additional hardware for flue gas recirculation and has a narrow operating range in terms of effluent oxygen content and flame stability. Achievable NO.sub.x levels with this burner design is a NO.sub.x emission level of about 45 to 60 ppm.
U.S. Pat. No. 4,505,666 issued to Martin, et al. teaches a staged fuel/staged air low NO.sub.x burner which involves creating two combustion zones. The first is created by injecting 40 to 60 percent of the fuel with 80-95 percent of the air, the second by injecting 40-60 percent of the fuel with 5-20 percent of the total air. Achievable NO.sub.x levels with this design have been shown in the 40-50 ppm range. There is no provision for utilizing flue gas recirculation.
U.S. Pat. No. 4,629,413 issued to Micheson et al. discloses a low NO.sub.x premix burner which delays the mixing of secondary air with the combustion flame and allows cooled flue gas to recirculate. A primary air system uses a jet eductor to entrain combustion air and mix it with fuel to pass the air/fuel mixture to a centrally disposed burner tip to be burned. A secondary air system dispenses air from an annular space formed about the burner so that secondary air is fed to the combustion flame, causing a longer time for secondary air to reach the fuel and thus lowering the peak flame temperature. Further cooling to the flame occurs as a result of small amounts of flue gas being entrained into the base of the less than stoichiometric, fuel rich flame, providing cooling and dilution of the flame. The patent shows a NO.sub.x emission level of between about 40 to 120 ppm (corrected to 4% excess oxygen on a dry basis).
With the exception of the Michelson et al. U.S. Pat. No. 4,629,413, the adverse effects of internally recirculated flue gas on flame stability have been avoided. The internal flue gas in a furnace, created by thermal gradients such as in a tubular furnace, is known to recirculate downwardly or back to the burner to interact sufficiently with the flame to cause flame instability or deformation. This deleterious backwash of flue gas was widely recognized and finally obviated by the inclusion of a flue gas deflection barrier which surrounded the burner at a height and spatial orientation to cause the internally recirculated flue gas in the furnace to be diverted away from direct interaction with the flame near the burner. This deflection barrier is well known as a Reed wall.
While NO.sub.x emission control by the above described prior art processes and apparatuses has generally proved satisfactory, tighter governmental restrictions are requiring ever improved performances beyond the capability of some of these burner assemblies, and in some instances, even where the prior art is technically capable of achieving the lower permissible NO.sub.x emission levels, the capital investment and/or increased operating expenses restrict their applications. There is a need, not only with regard to new installations, but also with regard to retrofit applications, for tighter NO.sub.x emission control which minimizes capital outlay and ongoing maintenance and operation expense.
That is, while heretofore known prior art processes and apparatuses are generally capable of reducing NO.sub.x emission levels, numerous disadvantages or limitations are presented by such prior art. The heretofore known prior art processes and apparatuses variously fail to provide full emission control; incur substantial downtime due to complexity of equipment; require addition of objectionable chemicals such as ammonia; or lead to additional emission constituents that are themselves recognized as undesirable. Further, the additional costs, including initial capital outlay and ongoing operating expenses, and the liability exposure presented by the heretofore known prior art processes and apparatuses are undesirable.